And in a veiled threat to Germany, Mr Bush said resistant nations such as them will have to make their own decisions as to "how, if, and when they want to participate".
"The world needs the nations of this continent to be active in the defence of freedom; not inward-looking or isolated by indifference," he said.
On the eve of a NATO gathering guarded by American warplanes overhead and overshadowed by the Iraq crisis, summit host Vaclav Havel, the Czech president, said his people prefer that Saddam Hussein peacefully surrender weapons of mass destruction.
"If, however, the need to use force were to arise, I believe NATO should give honest and speedy consideration to its engagement as an alliance."
NATO Secretary-General George Robertson, previewing a gathering to expand and modernise the alliance, predicted there will also be "total unity of the heads of state and governments on support for the UN Security Council resolution" on Iraq. But it's too early to say what that support would mean for NATO, Robertson added.
"Even in this beautiful city, I don't think it is wise to cross bridges before you come to them," he said.
On the cobblestones of picturesque Old Town square, several hundred demonstrators thousands fewer than were threatened protested at the summit that convenes tomorrow.
Among them were about three dozen leftists whose banners read: "No war in the name of democracy" and "Don't drop bombs! Drop Bush!"
Tucked into the security of a locked-down hotel across town, Bush told students that "great evil is stirring in the world".
"We've faced perils we've never thought about, perils we've never seen before, but they're dangerous, they're just as dangerous as those perils that your fathers and mothers and grandfathers and grandmothers faced."
Bush invoked the US involvement in World War II as he exhorted allies to join America now. "U-boats could not divide us," Bush told the students, who sat silent through his speech. "The commitment of my nation to Europe is found in the carefully tended graves of young Americans who died for this continent's freedom."
During a separate meeting with Havel earlier in the day, Bush sought to quiet European fears that he is hungry for war. He promised consultation with allies, saying a military clash with Iraq was his "last choice" and an avoidable one.
It is still possible Saddam could get the message, Bush said. "If the collective will of the world is strong, we can achieve disarmament peacefully."
Aides said afterwards that Bush still believes it is highly unlikely that Saddam will comply, thus war planning is fast under way. Bush discussed Iraq behind closed doors with President Ahmet Necdet Sezer of Turkey, whose country shares a border with Iraq and has crucial military bases.
Bush had no plans to meet with German Chancellor Gerhard Schroeder, whose recent re-election campaign infuriated the president by focusing on opposition to Bush's Iraq policy.